First of all, you will need to decide upon a narrower topic regarding Tennessee Williams's The Glass Menagerie. So, perhaps by going to the links here on enotes, you can read about theme, characters, setting, etc., and decide. There are also on enotes some excellent essays written on the play by critics. In addition, you can peruse the Question and Answer and Discussion Groups under The Glass Menagerie as there have been some interesting questions asked which may help "jump start" your ideas.
For instance, there are a number of questions regarding the use of symbols, including the religious symbolism in the final scene [candles, madonna, Jim as savior, etc]. Or, you can choose to analyse a character or discuss one of the motifs, such as how Williams's play is a coming of age play despite the fact that Laura and Tom are already adults.
Regarding the writing of your thesis, there is also help here on enotes. (See the links below.) In the thesis, you lay the blueprint for your essay, so be sure to limit it to three significant points that you can fully support. Providing "proof" is the aspect that truly makes for a strong essay; so you will wish to use many citations from the play. For instance, if you write on Laura, you could explain how she is crippled physically, emotionally, and mentally.